1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Related Art
Since dot gain of a printer of an electrophotographying system is large in comparison with a printer using an ink, a character may be printed thicker than intended when a same font character is printed. Accordingly, a process for making the line width of the character thin (hereinafter referred to as a thinning process) is performed by an image process (For example, see Japanese Patent Laid-Open No. 2005-341249). The thinning process is a process that evenly lowers the pixel value of outline portion of the character by converting it to a halftone level by capturing an edge of an image of a solid character (a character having a pixel value of a character portion being near the maximum value). Thereby, the solid character easy to become thick can seem thinly.
When the character is a halftone character (a character having a pixel value of a character portion being halftone), the jaggy generated by a screen process is a problem than becoming thick about the character. The edge emphasis process may be performed to reduce the jaggy. However, since the edge emphasis is performed to additionally draw the outline, there is tendency that makes the character thick. Additionally, the thinning process is performed to the solid character as disclosed in Japanese Patent Laid-Open No. 2005-341249, and on the other side, the edge emphasis process is performed to the halftone character. When comparing both the processes, an unbalance result may occur because the halftone character seems more thickly.
Especially, the tendency becomes remarkably when a black character is printed. With respect to a same black solid character, there is the case that it is outputted by a black monochrome or there is the case that it is outputted by four colors of C (cyan), M (magenta), Y (yellow), and K (black). For example, when a document prepared by word processing software is printed directly, the black solid character is printed by the black monochrome. However, when the document is converted to other file type such as PDF, and so on once, and printed, the black solid character may be printed using four colors of CMYK. Also, there is a possibility that the same case may occur when the setting is changed in a printer driver software.
In a document representing a black using four colors, it is considered to perform both the thinning process and the edge emphasis process. In order to represent the black, generally, each image of each color of CMYK is outputted at a density of 20-30% degree to overlap the images. That is, since each color image is halftone, the edge emphasis is performed without thinning it.
FIG. 11 are drawings showing that a screen process and/or an edge emphasis process is/are performed to an original image of each color when a black character is represented by overlapping halftone images of the four colors of CMYK. FIG. 12 is a view seen from an upper surface of a sheet after overlapping images of each color and a sectional view after printed. As shown in FIG. 11, since the jaggy occurs when the screen process is performed to a halftone image, it is necessary to perform the edge emphasis process. However, as shown in FIG. 12, since the outline is drawn by each color of CMYK when the edge emphasis is performed, toners of each color are always deposited in the outline portion of the character at the time of printing. Therefore, as shown in the sectional image in FIG. 12, the outline portion extends and the character becomes thick. That is, there is a possibility that the edge of the character is emphasized more than required. According to this, the tendency that the whole black character becomes thick occurs.
In contrast, as shown in FIG. 13, when black is represented by the black monochrome, the black is outputted by a high density near 100% being the maximum value. In this case, since the thinning process is performed to prevent that character become thick, the tendency that the character becomes thin occurs as shown in FIG. 13.
According to this, the inconvenience that a certain character becomes thin or a certain character becomes thick occurs according to whether it is outputted by the black monochrome or the four colors of CMYK in spite of the same font character.